In the context of its broadest general application, the term ‘gasification’ has generally been used to describe any process for the conversion of a solid, liquid, or vapor hydrocarbon or more heterogeneous but predominantly carbonaceous compounds into synthesis gas (also referred to as syngas). The synthesis gas generally consists of a mixture of gases consisting predominantly of carbon monoxide and hydrogen. Low quality synthesis gas may be contaminated by methane, CO2, and other impurities such as, but not limited to, aromatics and high molecular weight tars. These tars, a common byproduct of biomass gasification, can be particularly undesirable since, aside from representing a syngas yield loss, such tars can lead to serious fouling and plugging of downstream process equipment when cooled below their dewpoint as required for downstream processing. When a gasification type process is applied as a secondary reaction stage for the conversion or removal of such impurities from a low quality synthesis gas, the term ‘conditioning’ is often used to describe such a secondary operation. Gasification or conditioning may therefore entail a wide variety of generic chemical reactions, which may include, without limitation, pyrolysis, cracking, partial oxidation, reforming, and/or shifting.
Depending on the particular carbonaceous feedstock, the impurities therein, and the specific reactions involved, such reactions may also entail the addition of steam and/or oxygen as a supplemental reactant(s) to help promote the desired reaction. Such processes may be conducted with or without the use of a catalytic or inert medium for the purpose of promoting efficient heat and mass transfer within the gasification reactor. Catalytic media may be particulate or monolithic in nature. A common drawback of some conventional gasification and proposed conditioning processes is byproduct soot formation, which can be even more problematic than tars if sufficiently ‘sticky’ to foul downstream process equipment at even elevated temperature levels.
Gasification with pure steam in a fluidized bed is a highly endothermic process. For this reason, gasification, for example biomass gasification, has been combined with fluidized bed combustion to provide heat enthalpy and also to remove char formed during gasification. Such char may be undesirable in the gasification producer gas. Dual fluidized bed gasification has thus been proposed in the art. Dual fluidized bed gasification is desirable due to the ability to produce high caloric product gas free of nitrogen dilution even when air is used to generate, via in situ combustion, the heat required by the endothermic gasification reactions.
Conventional gasification processes may be generally classified as either ‘direct’ or ‘indirect’. In direct gasification processes an oxidant is directly contacted with a carbonaceous feed. In indirect gasification processes, an oxidant is supplied to a separate combustion reactor and is kept separate from the carbonaceous feed in the gasification reactor by a physical barrier which may also serve as a heat transfer medium. The medium transfers heat from the exothermic combustion reactor to the endothermic gasification reactor, as exemplified by dual fluid gasification which employs a circulating solid heat transfer medium. Another type of indirect gasification technology uses stationary heat transfer tubes to separate and transfer heat between the endothermic gasification and exothermic combustion reaction sections.
Direct gasification, which is currently widely practiced, generally utilizes three basic configurations which may be either air blown or oxygen blown: entrained flow (e.g. Siemens), fluidized bed (e.g. Winkler), and moving bed (e.g. Lurgi dry bottom). When air blown, the nitrogen component of the air undesirably dilutes the product synthesis gas, rendering it unsuitable for direct use in various applications. For this reason, many direct gasifiers are oxygen-blown, requiring a source of high purity oxygen, which tends to be expensive. For example, an air separation unit is often utilized to provide the oxygen for an oxygen-blown gasifier. Despite the use of high purity oxygen, direct gasification, especially via fluid bed and moving bed gasifiers, often provides low-quality synthesis gas which, while appropriate for many applications, may be undesirable for processes demanding high-quality synthesis gas. While oxygen blown entrained flow gasifiers may produce a better quality synthesis gas than produced by fluid bed or moving bed configurations, entrained flow gasifiers generally require more stringent feedstock preparation which may not be practical for certain types of carbonaceous feeds, such as biomass or petroleum coke. Even if free from impurities, the synthesis gas produced by entrained flow or other types of gasification technologies may not meet a desired composition for certain downstream chemical or fuel synthesis applications. Further conditioning of such synthesis gas may therefore be required.
Indirect gasification technologies, particularly those based on dual fluidized beds, are generally known to produce low-quality synthesis gas comprising undesirably large amounts of impurities such as hydrogen, methane, carbon dioxide, and high dew point tars. Such technologies also generally mandate consumption of high levels of steam and other additives such as dolomite to promote gasification and maximize levels of quality product synthesis gas. Typically, undesirable components such as methane, carbon dioxide, excess hydrogen, tars, and/or sulfur and sulfur-containing components must be removed from low-quality synthesis gas produced via gasification prior to the use of the synthesis gas in processes requiring chemical-grade synthesis gas. This contaminant removal may be costly, inefficient and complicated. The presence of such contaminants may also represent a substantial loss of potential product synthesis gas and downstream product yield if such contaminants are not converted to the high quality syngas required for certain chemical and fuel production processes.
Accordingly, there is need for systems and methods of producing high-quality synthesis gas from gasification processes, especially from indirect gasification technologies such as dual fluidized bed gasification. Desirably, such systems reduce or eliminate the need for extensive downstream cleaning of the synthesis gas produced, allow for production of synthesis gas in the absence of costly air separation unit(s), reduce/eliminate potential byproduct soot formation and/or provide higher yields of synthesis gas of a desired composition than conventional gasification systems and methods.